This invention relates to the meltblowing of thermoplastic materials to produce nonwoven fibrous fabrics. In particular the invention relates to an improved airless water quenching system for rapidly cooling the extruded fibers of the fabric.
Meltblowing is a process wherein a molten thermoplastic material is extruded through a meltblowing die to form a plurality of side-by-side molten fibers. Convergent jets of heated air are directed onto opposite sides of the extruded fibers to attenuate the fiber diameters by stretching the fibers. The air jets and fibers form a fiber and air stream which is directed onto a moving collector surface where the fibers randomly deposit to form a nonwoven fabric or web. The web is held together primarily by fiber entanglement with some fiber-to-fiber sticking while still in the molten or semi-molten state. Nonwoven webs so produced have a number of commercial uses including medical products and filters, absorbents, and battery separators to name a few.
Efforts to improve the economics of the meltblowing process have focused on increasing the web production rate (polymer throughput) without compromising the final properties of the web, as well as adding significantly to the production costs. Important mechanical properties of the web include strength, stretchability, and absorbency. Equally important in some applications are the fabric-like properties of hand and drape. In the past it has been found that simply increasing the polymer throughput in the die led to the formation of polymer agglomerations known as "shot". The shot forms in the fiber and air stream because the increased polymer throughput results in larger diameter fibers which do not have sufficient time to cool and solidify into individual fibers before fusing with adjacent fibers, whereby the fibers tend to agglomerate. The formulation of shot degrades virtually all of the desirable properties of nonwoven webs. High quality meltblown webs are held together by gross mechanical entanglement with only a small amount, if any, of interfiber fusion or sticking.
It is known in the art of meltblowing to apply a quenching fluid to the molten fibers shortly after leaving the die to rapidly cool and solidify the fibers, whereby adjacent fibers do not agglomerate and form shot. The use of a quenching fluid permits a higher polymer throughput while avoiding the problem of shot formation and fabric degradation.
An equally important aspect of quenching is the effect on web properties. It has been shown experimentally that the application of quenching may significantly improve the web properties such as hand, stretchability, and tear strength. This is true even in situations where shot formation is not a problem as in fine fibers. Meltblown fabrics having fine fibers are important in medical and filtration applications. Quenching alters the web properties by influencing the polymer fiber crystallization and solidification rates. Quenching also minimizes interfiber fusion so that the fabric is held together almost exclusively by fiber entanglement.
U.S. Pat. No. 3,959,421 discloses a conventional meltblowing die and collector apparatus with water spray nozzles positioned therebetween. The nozzles are air-driven nozzles which use compressed air to break up, or atomize, liquid water to produce a spray of water droplets. The droplets are directed onto opposite sides of the extruded fibers at a position upstream of that where shot would form in an otherwise unquenched process. The water droplets cool the fibers primarily through evaporative cooling wherein the liquid water droplets are vaporized, thereby removing the latent heat of vaporization from the fibers. The Patent discloses that the application of quenching fluid allows a production rate of up to about 3.0 lb/hr/in. The production rate is limited by the maximum allowable quench water rate with-out the formation of what is referred to as a "wet" web. A wet web is one which has liquid water entrapped within voids in the fabric. The Patent further discloses experimental data for polypropylene which shows a significant improvement in web properties when quenching is applied.
U.S. Pat. No. 4,594,202 discloses the use of air-driven water atomizer nozzles for use in producing tubular meltblowns having fiber diameters in excess of 2.5 microns.
From the foregoing it is apparent that improvements in the application of an evaporative quenching are important for improving the economics of the meltblowing process as a whole, both from the standpoint of eliminating shot at high production rates, as well as improving web properties over a range of production rates.
A problem encountered in prior art water quenching nozzles is associated with the use of air-driven nozzles to atomize the liquid water into droplets. This type of nozzle requires both a source of compressed air and a source of liquid water which must be metered out in precise volumetric relation to achieve a uniform spray at the desired rate. In longer dies, this approach may require the use of numerous nozzles which in general must each be adjusted and metered independently. For example, the use of thirty nozzles may require the metering of nearly sixty flow lines, one water, and one air for each nozzle. Air operated nozzles also tend to form relatively large water droplets. Moreover, the droplet size of the air or atomized spray is much larger than that used in the present invention. For example, air atomized nozzles produce droplets ranging in size from about 20 to about 60 microns. The inertia of the larger droplets presents problems of disrupting the polymer filament pattern from the die to the collector.
It is an object of the present invention to provide a simplified quenching apparatus which provides exceptional fiber cooling when applied to meltblowing.